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Nanoscale Iron Could Help Cleanse The Environment; Ultrafine Particles Flow Underground And Destroy Toxic Compounds In Place

Source: National Science Foundation
Date:
2003-09-04

Nanoscale Iron Could Help Cleanse The Environment; Ultrafine Particles Flow
Underground And Destroy Toxic Compounds In Place
Arlington, VA - An ultrafine, "nanoscale" powder made from iron, one of the
most abundant metals on Earth, is turning out to be a remarkably effective
tool for cleaning up contaminated soil and groundwater--a trillion-dollar
problem that encompasses more than 1000 still-untreated Superfund sites in
the United States, some 150,000 underground storage tank releases, and a
staggering number of landfills, abandoned mines, and industrial sites.

The case for nanoscale iron is laid out in the September 3 issue of the
Journal of Nanoparticle Research, where Lehigh University environmental
engineer Wei-xian Zhang reviews his eight years of pioneering work with the
material. Much of Zhang's research has been funded by the National Science
Foundation as a part of the federal government's 16-agency National
Nanotechnology Initiative (NNI). This issue of the Journal is dedicated to
nanoparticles in the environment and it is prefaced by Mihail Roco, NNI's
coordinator and NSF's Senior Advisor on Nanotechnology, with a perspective
on "Broader Societal Issues of Nanotechnology".

Iron's cleansing power stems from the simple fact that it rusts, or
oxidizes, explains Zhang. Ordinarily, of course, the only result is the
familiar patina of brick-red iron oxide. But when metallic iron oxidizes in
the presence of contaminants such as trichloroethene, carbon tetrachloride,
dioxins, or PCBs, he says, these organic molecules get caught up in the
reactions and broken down into simple carbon compounds that are far less
toxic.

Likewise with dangerous heavy metals such as lead, nickel, mercury, or even
uranium, says Zhang: The oxidizing iron will reduce these metals to an
insoluble form that tends to stay locked in the soil, rather than spreading
through the food chain. And, iron itself has no known toxic effect--just as
well, considering the element is abundant in rocks, soil, water, and just
about everything else on the planet. Indeed, says Zhang, for all those
reasons, many companies now use a relatively coarse form of metallic iron
powder to purify their industrial wastes before releasing them into the
environment.

Unfortunately, says Zhang, these industrial reactors aren't much help with
the pollutants that have already seeped into the soil and water. That's the
beauty of the nanoscale iron particles. Not only are they some 10 to 1000
times more reactive than conventional iron powders, because their smaller
size collectively gives them a much larger surface area, but they can be
suspended in a slurry and pumped straight into the heart of a contaminated
site like an industrial-scale hypodermic injection. Once there, the
particles will flow along with the groundwater to work their decontamination
magic in place--a vastly cheaper proposition than digging out the soil and
treating it shovelful by shovelful, which is how the worst of the Superfund
sites are typically handled today.

In that sense, says Zhang, nanoscale iron is similar to in situ biological
treatments that use specialized bacteria to metabolize the toxins. But
unlike bacteria, he says, the iron particles aren't affected by soil
acidity, temperature, or nutrient levels. Moreover, because the
nanoparticles are between 1 and 100 nanometers in diameter, which is about
ten to a thousand times smaller than most bacteria, the tiny iron crystals
can actually slip in between soil particles and avoid getting trapped.

Laboratory and field tests have confirmed that treatment with nanoscale iron
particles can drastically lower contaminant levels around the injection well
within a day or two, and will all but eliminate them within a few
weeks--reducing them so far that the formerly polluted site will now meet
federal groundwater quality standards. The tests also show that the
nanoscale iron will remain active in the soil for 6 to 8 weeks, says Zhang,
or until what's left of it dissolves in the groundwater. And after that, of
course, it will be essentially undetectable against the much higher
background of naturally occurring iron.

Finally, says Zhang, the cost of the nanoscale iron treatments is not nearly
as big a barrier as it was in 1995, when he and his colleagues first
developed a chemical route for making the particles. Then the nanoscale iron
cost about $500 a kilogram; now, it's more like $40 to $50 per kilogram.
(Decontaminating an area of about 100 square meters using a single injection
well requires 11.2 kilograms.)

Zhang is currently forming a company to mass-produce the nanoscale iron
particles. And in the meantime, he and his colleagues are consulting with
multiple clients. "It used to be just the feds--agencies like the Navy and
so on," he says. "But now, we're working with big pharmaceutical firms,
semiconductor manufacturers, and many other companies, all of which are
interested in cleaning up sites."

After nearly a decade of research, he adds, "we're entering a phase of
exponential growth. There are thousands and thousands of contaminated sites
out there. And hopefully, this will be a cost-effective way to deal with
many of them."